Islamabad: US drones fired missiles on militant compounds in the lawless North Waziristan tribal region on Friday killing at least 12 insurgents, the latest in a series of such attacks termed by Pakistan as unlawful and a violation of its sovereignty.

The CIA-operated spy planes targeted compounds and a vehicle in three strikes in Shawal region of North Waziristan tribal agency, which has witnessed a slew of drone attacks in the past few weeks.

Twelve militants were killed in the strikes, officials were quoted as saying by TV news channels.

The drones fired six missiles at the targets. The spy planes continued hovering over the area after the attacks.

Today's attacks came a day after Pakistan's Foreign Office summoned a senior US diplomat to protest a string of drone strikes since last week, saying the attacks by the spy planes were unlawful and a violation of the country's sovereignty.

The US embassy was "demarched on recent drone strikes in North Waziristan. A senior US diplomat was called to the Ministry of Foreign Affairs and informed that the drone strikes were unlawful, against international law and a violation of Pakistan's sovereignty," said a brief statement issued by the Foreign Office spokesman.

"It was emphatically stated that such attacks were unacceptable," the statement said.

Over 20 militants, a majority of them members of a Taliban faction led by Hafiz Gul Bahadur, have died in four earlier drone strikes in North Waziristan since August 18.

Ten militants were killed in two drone strikes in North Waziristan on August 19. Most of the attacks were carried out in Shawal area.

The region is dominated by Hafiz Gul Bahadur's fighters, who often target US and other foreign troops in Afghanistan.

The US has been pushing Pakistan to act against Taliban and al-Qaeda elements in North Waziristan.

US Defence Secretary Leon Panetta recently claimed that Pakistan was preparing an operation against the Taliban in North Waziristan.

However, Pakistan Army Chief Gen Ashfaq Parvez Kayani has said his forces will undertake an operation in North Waziristan "at a timeframe of our choosing and determined only by our political and military requirements."